check if invoice is already paid

[Waqas Saeed]

the most simples question,

do I have alternative to this, if WHMCS has some built-in function to check if the invoice is already paid and redirect to the URL ? 

$results = localAPI($command, $postData, $adminUsername);
$getInvoice = $results['status'];

if($getInvoice=="Paid") { ?>
        <script language="javascript">
        window.location.replace("<?php echo $systemUrl.'viewinvoice.php?id='.$invoiceId.'&paymentsuccess=1' ?>");
        </script>
<?php }

Another, What is the name of the function that check if the user belong to that invoice specifically.?